Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 3: The Foundation Statements of T-SQL
While SQL Server is nice enough to let us UPDATE pretty much any column (there are
a few that we can’t, such as timestamps), be very careful about updating primary
keys. Doing so puts you at very high risk of orphaning other data (data that has a reference
to the data you’re changing).
For example, the StoreCode field in the Stores table is a primary key. If we decide
to change StoreCode 10 to 35 in Stores, then any data in the Sales table that relates to
that store may be orphaned and lost if the StoreCode value in all of the records relating
to StoreCode 10 is not also updated to 35. As it happens, there is a constraint that references
the Stores table, so SQL Server would prevent such an orphaning situation in
this case (we’ll investigate constraints in Chapter 7), but updating primary keys is
risky at best.
The DELETE Statement
The version of the DELETE statement that we’ll cover in this chapter may be one of the easiest statements
of them all. Even in the more complex syntax, there’s no column list, just a table name and (usually) a
WHERE clause. The full version looks like this:
DELETE [TOP ( ) [PERCENT] ] [FROM]
[ OUTPUT ]
[FROM ]
[WHERE | CURRENT OF [GLOBAL] ]
The basic syntax couldn’t be much easier:
DELETE
[WHERE ]
The WHERE clause works just like all the WHERE clauses we’ve seen thus far. We don’t need to provide a
column list because we are deleting the entire row. (You can’t delete half a row, for example.)
Because this is so easy, we’ll perform only a couple of quick DELETEs that are focused on cleaning up the
INSERTs that we performed earlier in the chapter. First, let’s run a SELECT to make sure the first of those
rows is still there:
SELECT *
FROM Stores
WHERE StoreCode = ‘TEST’;
If you haven’t already deleted it, you should come up with a single row that matches what we added
with our original INSERT statement. Now let’s get rid of it:
DELETE Stores
WHERE StoreCode = ‘TEST’;
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